Ci migration doc improvements

Use export improvements in CI 8.0.1, make upgrades a more prominent part of the process, do not shut down GitLab during CI data import.

See merge request !1387

documentation on ssl verification in web hooks



See merge request !1386

Fix CI permissions

![Screen_Shot_2015-09-21_at_20.23.16](https://gitlab.com/gitlab-org/gitlab-ce/uploads/6f40699507d0dce148d0054a093f0d63/Screen_Shot_2015-09-21_at_20.23.16.png)

This fixes two problems:
- Allow developers to retry builds
- Hide advanced project options from CI page for non-admin users



See merge request !1379

Add missing rake and change nginx config for CI migration

/cc @marin

See merge request !1385

Reformat and copy edit the CI-to-CE migration guide

[ci skip]

See merge request !1335

[ci skip]

Tweak remaining CSS for Markdown preview

See merge request !1383

- Rename "Finishing Up" to "Nginx configuration"
- Add fourth "Finishing Up" section
- Add Troubleshooting section with a link to backup restoration docs
- Change a few indented blocks into fenced blocks so they're highlighted
  properly.
- Added Marin's suggestions for CI-to-CE-on-different-server section.

Missed a few items in !1378

Ensure raise_error expectations provide arguments

See merge request !1381

Fix Error 500 when comparing non-existing branches

Closes #2593

See merge request !1355

Remove deleted milestones from merge requests

This is related to !1348 but thought it would be useful as a separate MR. This MR removes deleted milestone IDs from any merge request.

Closes https://github.com/gitlabhq/gitlabhq/issues/9643

See merge request !1350

Allow relative links to go up one directory level

See merge request !1352

Closes https://github.com/gitlabhq/gitlabhq/issues/9643

Rename reply_by_email to incoming_email to prepare for the future.

Closes #2579.

See merge request !1367

CI import improvements



See merge request !1369

Fixed MR handling when GitLab CI project is not present

This is copy of !1371

/cc @jacobvosmaer @vsizov @dzaporozhets



See merge request !1373

- Allow developers to retry builds
- Hide advanced project options from CI page for non-admin users